This inventions is related to the measuring of complex signal internal relationships, more particularly, to measuring the horizontal sync pulse to subcarrier phase relationship of video signals.
Video signals are a composite of various pulses and selectively superimposed subcarrier signals. One element of these signals is a series of sync pulses of which there are two types--vertical and horizontal. Vertical sync pulses are broad pulses which occur every so many lines to trigger the TV (television) to go from the bottom of the picture to the top. The horizontal sync pulse is the trigger which resets the scan to the left side of the picture after each horizontal line is completed.
In the U.S. there are 525 lines in a TV picture. These are divided into two interlaced fields with one-half of the lines being scanned in the first field and the other half of the lines being scanned in the second field. In each field 262.5 lines are painted on the screen with one-half line at the end of the first field and one-half line at the beginning of the second field. Thus, to complete a single picture, two fields are necessary and the paired fields are called frames.
This relationship was defined by the EIA (Electronic Industries Association) in standard RS-170 in the 1940's. As defined therein, the frame repetition rate is 30 Hz.
Next following the horizontal sync pulse in the video signal is a subcarrier reference color burst. The burst is followed by an analog signal varying from the black to white level with color information thereon in phase modulated side bands of the superimposed subcarrier signal.
In recent years EIA has proposed a color standard, RS-170A wherein the relationship between horizontal sync and the subcarrier is specified. In RS-170A the subcarrier frequency has been selected to be 3.579545 MHz and the horizontal scanning frequency is defined to be EQU H=(2.multidot.SC/455) (1)
where H is the horizontal sync frequency and SC is the subcarrier frequency stated above.
As a result of this definition and the fact that there are 525 lines per frame, there are 227.5 cycles of subcarrier per line resulting in the phase of the subcarrier alternating every line. Additionally, there are 119437.5 cycles of subcarrier per frame. Therefore, because of the extra half cycle of subcarrier per frame, it takes two frames to complete a color sequence.
Also, because of the relationship of equation 1, the horizontal sync frequency is locked to the subcarrier frequency, but this does not define the phase relationship of the sync and subcarrier signals. RS-170A defines the phase relationship between these two signals as being in phase when the zero crossing of the extrapolated subcarrier of burst aligns with the fifty percent point of the leading edge of sync as shown in FIG. 1. This definition of horizontal sync (H) to subcarrier (SC) phase (SC/H phase) is required for the unambiguous identification of the four field sequence (color frame) where a color frame consists of a pair of two field frames.
The SC/H phasing is important for two reasons, first because of the possible future adoption of RS-170A by the FCC and secondly in the video tape environment. A video tape machine in a normal synchronized playback mode, where the machine locks to house sync (studio master timing clock generator), has a sequence of three steps to achieve lock. First, the machine frame locks by aligning a control track frame pulse with a frame pulse derived from house sync. There are two control track frame pulses per color sequence on the first field to each frame. The second step is locking horizontal sync to the house reference. Third to lock is the color time base corrector, in order to make the subcarrier correct, the horizontal phase of tape sync must be moved with respect to the house reference. This error can be as great as 140 ns, or one-half cycle of subcarrier. The case of 140 ns error implies that the video on the tape is of the other color field as compared to house reference. Due to the necessity of this locking sequence the tape machine makes frame assignment between house and tape sync first without regard to the correct frame in the color sequence. On playback mode there are even odds that the machine will lock to the incorrect frame. Under this condition, sync, that is on the tape, will be 140 ns (half a subcarrier cycle) out of time with house sync. This will result in a shrinking of active picture and a widening of horizontal blanking width since the output processor of the tape machine is inserting blanking referenced to house sync. Some tape machines have a color framer, which essentially adds a fourth step in the locking sequence. If upon horizontal lock the horizontal phase error between tape video and input video is greater than 70 ns, the machine will assume it is locked to the incorrect frame, and will shift frames and again complete the locking sequence. One disadvantage with color frame lock is that it can take twice as long to lock. A different approach is with a 15 Hz framer. When video is recorded on the tape only every other frame pulse is recorded, thus when the tape is moved from one tape machine to another, the relationship between the frame pulse on the control track and the house color frame is lost.
This horizontal shifting becomes critical in a tape editing environment especially while editing scenes together of similar content. During the edit the similar background will appear to jump horizontally. A specific example of this is in animiation where several items are to be popped on or off the screen against the same background.
To insure the proper operation of the tape machine color framer, the SC/H phase relationship of the video recorded on tape, and that of house sync and subcarrier must be matched. For uniformity, correct SC/H phase is defined by RS-170A. It is therefore best for all video recorded on tape to have a consistantly correct SC/H phase relationship, and the input video to the tape machine also to be stable and to have correct SC/H phase.
The concept of timing a studio for subcarrier is well understood in the industry, but timing a house for both sync and subcarrier requires a new level of thought regarding each element within a studio. First and most obvious is the house sync generator. If the sync generator cannot generate consistant SC/H phased signals, there is no way to SC/H phase a plant. It is equally important in a multiple sync generator plant, that all generators maintain the SC/H and color frame relationships. Once SC/H phase has been defined by the sync generator all other elements in the system should not alter the SC/H phase relationship. This implies many things. Some obvious elements are processors, which regenerate sync and burst. If the phase of the regenerated sync or burst is different from the incoming video, the SC/H phase will be altered. Less obvious are sources which derive timing from externally applied sync and subcarrier. If these pulses are fanned out through DAs, then both sync and subcarrier timing can be altered independently. This implies that the output of each source must be timed for both sync and subcarrier to the input of the switcher.
There are many distortions which make difficult the determination of color frame and SC/H phase. First is sync to subcarrier time base errors. This can be generated by many elements, such as sync generators with noise in their horizontal sync circuits, linear and regenerative pulse DAs which suffer from pick off jitter or low frequency response problems, or any processor or source equipment that can effect the sync to subcarrier phase relationship.
There are also problems in signal transmission, such as noise which makes the determination of the exact fifty percent point of sync difficult, or low frequency response problems creating smear, which creates distortions to the leading edge of sync. Lastly, is hum or power line glitches which are not removed prior to sync separation.
The sync to subcarrier time base error is different than the entire video time base error. Sync to subcarrier time base error may be checked by triggering a scope on the 50 percent point of the leading edge of sync, and viewing the burst. What should be seen are two overlapping cycles of subcarrier which are not blurred.
If sync to subcarrier time base error occurs either on the reference pulses to a tape machine or exists on the video recorded on the tape, it makes color frame lock difficult. In the normal synchronized lock mode, if either time base error condition is excessive, the tape machine will shift horizontal lines by 279 ns (one subcarrier cycle) increments. This phenomenon is seen as a tearing of the picture.
To achieve an SC/H phased plant, the timing of sync becomes as important as subcarrier, and each element should be viewed in that light. To aid video tape editing, it is important to record video with proper SC/H phase and also supply SC/H phased video to the machine in playback.
The measurement of SC/H phase is not a simple proposition without a specialized instrument. To do so one needs the following equipment as well as a good working knowledge of what one is looking at and for:
______________________________________ Dual Trace Oscilloscope Tektronix 465 (with delayed sweep and or equivalent one channel input inversion) Switchable Delay Line Matthey 511 or equivalent or- Continuously Rotatable Subcarrier GVG (Grass Valley Group) (360 degree phase) 3407 DA or equivalent ______________________________________
There are two units on the market currently which attempt to measure the SC/H phase but for various reasons fall short of doing a complete job.
One such unit is manufactured by Leitch which has a resolution of approximately 9.degree.. However, this unit utilizes only a single video input signal which makes it impossible for the unit to do a color frame comparison to insure that the color fields are also properly aligned.
The second unit is manufactured by Lenco. This unit utilizes two video input signals which enables it to measure SC/H phase, as well as, do a color frame comparison. However, this unit requires that the two video input signals be applied at exactly the same time which dictates complex circuitry to provide the desired result. This unit is expensive and extremely hardware intensive.
What is needed is a reliable, simple, easy to use piece of equipment which not only measures SC/H phase, but also addresses the color frame comparison issue to insure the quality of the video signal throughout the house system without the requirement that the input video signals be pre-timed, one with respect to the other. It is believed that the current invention provides such an instrument.